The technique for recovering the considerable quantity of heat from the exhaust gas by using a regenerator to preheat the air for combustion has been recently developed. For example, a radiant tube burner having the configuration such as shown in FIG. 8 is proposed. In this burner, two burners 103 each having a regenerator 102 are fixed at both ends of a radiant tube 101, and combustion is alternately performed at the two burners 103. When combustion is effected at one burner 103, the produced combustion gas is exhausted through the regenerator 102 provided at the other burner 103 where no burning is made for preheating the air for combustion by using the heat of the combustion gas accumulated in the regenerator 102.
In such a regenerative alternate combustion burner system, the regenerator requires a flow switching apparatus for switching a flow through which the high temperature exhaust gas flows to another flow through which the low temperature air for combustion flows, and vice versa. Conventionally, a solenoid valve has been generally adopted as such a flow switching apparatus in the combustion system. For example, as shown in FIG. 8, the prior art apparatus has a configuration in which the solenoid valves 104, 105, 106 and 107 are provided at four positions and are selectively opened or closed to switch the flows for two kinds of gas having high and low temperatures. In the drawing, reference numerals 108 and 109 designate the solenoid valves for selectively opening or closing a fuel supply system 110.
The flow switching apparatus having the solenoid valves therein however requires a plurality of expensive solenoid valves, thus increasing the production cost. In particular, application of the apparatus to a heat exchanger in the combustion system also requires a number of more expensive solenoid valves for high temperature, then the production cost increases. Further, since the size of the solenoid valve for the air piping is considerably large and these four valves must be used in the system, they take up the large space and the double piping arrangement is needed, complicating the configuration. For instance, when such an apparatus is applied in a furnace or a soaking pit in an ironworks, thousands of solenoid valves are required in the apparatus. Moreover, the frequent switching operations between the air flow and the exhaust gas flow in a short period like one minute or less may result in deterioration of the durability of the solenoid valves.
In addition, the use of a four-way valve 207 shown in FIG. 9 may be also possible. The four-way valve 207 uses a directional control valve stem 206 which swivels in a valve casing 205 having four ports 201, 202, 203 and 204 in order to communicate any two adjacent ports with each other to switch the flows.
However, this four-way valve alternately leads the two types of gas each having a temperature greatly different from each other into the system. Any clearance must therefore be set between the directional control valve stem 206 and a main frame 205 of the valve to avoid the action failure due to the thermal expansion, and the short pass which may occur to the two flows in the valve in apt to lead to the gas leakage. For example, when using this valve in the illustrated regenerative radiant tube burner, since the air for combustion continuously leaks to the exhaust gas flow in the four-way valve 207 and the amount of leakage is inconstant and uncertain, there is a drawback that the air ratio in the combustion can not be securely controlled. Further, since the dimension of the four-way valve can not be increased, the use of this valve to the flows having a large capacity is not preferable.
As a conventional heat exchanger system for effecting heat exchange between two fluid systems via the regenerator, the Ljungstrom type air preheater shown in FIG. 10 is generatlly used. In the Ljungstrom type air preheater 400, two flows 402 through which the relatively high temperature gas such as the exhaust gas flows and two flows 403 through which the relatively low temperature gas such as the air for combustion flow are fixed to a casing 411. The rotation of a disc type regenerator 401 enables switching of the gas flows to the regenerator 401 without changing over the flow itself, and the air for combustion is preheated by using the heat recovered from the exhaust gas. In this Ljungstrom type air preheater 400, the upper and lower parts of the rotatable generator 401 are respectively divided into at least two chambers 406 and 407 and other two chambers 409 and 410 by partition walls 405 and 408 and a sealing material 404. Since the regenerator 401 is substantially divided into two areas by the sealing material 404, the regenerator 401 is heated by the exhaust gas in one area while the air for combustion is preheated in the other area.
When using the Ljungstrom type air preheater, however, the gas leaks from the high pressure (the low temperature gas) side to the low pressure (the high temperature gas) side. Although various means are adopted to the sealing in order to avoid this leakage, the sealing effect can not be obtained on the high temperature side before heat exchange, being impossible to suppress the leakage. About 25% of the air for combustion therefore leaks to the exhaust gas flow and the quantity of the leakage is inconstant. Hence there arises a problem that the air ratio can not be precisely controlled when directly using the preheated air to the combustion system. In addition, when employing the large regenerator, the rotational mechanism may have a enlarged structure or the strength of the regenerator may be unreliable due to the heavy weight of the regenerator.